The invention relates to a semiconductor array having a semiconductor element containing several successive narrow layers which are alternately n-doped and p-doped, to methods for maintaining the semiconductor array, and to its use.
Such structures are known, for example from the article by J. A. Cooper, JR. et al. in the periodical IEEE Electron Device Letters, EDL-3, No. 12, 1982, pages 407 and 408, and permit an increase in the mean drift speed of electrons in semiconductors. This speed increase is achieved by a step-like potential pattern provided in a semiconductor and shown in FIG. 1a. From this, a periodically structured electrical field in accordance with FIG. 1b builds up, comprising a sequence of narrow areas with high field strength (E.sub.1) and overlaid by a background field with low field strength (E.sub.0). This periodic field pattern subjects the electrons periodically to an acceleration, so that a wave-like pattern of the speed is generated as shown in the graph in FIG. 1c, thereby causing an increase in the mean drift speed.
The conduction mechanism underlying this phenomenon depends substantially on the band structure of the semiconductor used. The semiconductor material GaAs, a III/V connecting semiconductor, has a high electron mobility of approx. 1.8.times.10.sup.7 cm/sec, since the electrons in the gamma minimum have a low effective mass. The band structure of gallium arsenide does however have a side minimum which is only a few tenths of an electron volt above the gamma minimum. With sufficiently high fields the conducting electrons can pass over into this side minimum, so causing a considerable fall in electron mobility, since the electrons have in the side minimum an effective mass which is greater than that in the central gamma minimum by a factor of 5. If the electrons are now held by suitable measures in the central gamma minimum, they retain their high electron mobility, resulting in a high mean drift speed. This is achieved by the electrons losing energy by polar dispersion to polar phonons while having to scale the step-like potential structure, and thereby not being able to pass over into the side minimum.
The increase in the mean drift speed in silicon or germanium is due, unlike in gallium-arsenide, to differing values for the energy and pluse relaxation times. The electrons in these semiconductor materials are accelerated by the periodically occurring high field strength beyond the thermal balance value for the speed, before being decelerated back to the balance value in the subsequent weak field area by pulse and energy relaxation processes. These processes also include dispersion processes to phonons, where the dispersion frequency of these processes depends very strongly on energy. The electrons are conducted in these structures in a series of equivalent delta valleys of the band structure.
In the above publication, page 408, left-hand column, 2nd paragraph, electrical field strength profiles of this type are achieved by narrow areas, each having a layer thickness of 2 nm, with alternate n-doping and p-doping on their planes, and following each other successively, in an undoped semiconductor element, with an n-layer being combined with a subsequent p-layer spaced 20 nm from it to provide a layer pair. The distance between two such layer pairs is 400 nm. An array of this type is however technologically scarcely feasible; since it requires doped layer thicknesses of 2 nm as well as updoped semiconductor materials between these layers. In addition, this array has the drawback that space charges resulting from the current flow have a detrimental effect on the required field profile, i.e. on the step-like potential pattern necessary for the required effect.